There's no need for these to be non-const. Suggested by @IdanHo in
https://github.com/SerenityOS/serenity/pull/9904#discussion_r704960184.
Perhaps we can make more internal slots of these and other objects const
references as well, but that's a bit more involved as they are used by
various functions expecting non-const references.
This commit adds the ZonedDateTime object itself, its constructor and
prototype (currently empty), and the CreateTemporalZonedDateTime
abstract operation.